The disclosure of the following priority application is herein incorporated by reference: Japanese Patent Application No. 11-288399 filed Oct. 8, 1999.
1. Field of the Invention
The present invention relates to an exposure apparatus employed to transfer a pattern of an original such as a mask or a reticle (hereafter referred to as a mask) onto a photosensitive substrate such as a wafer during a photolithography process implemented in the fabrication of a semiconductor device such as an LSI or a semiconductor device in an image-capturing element such as a CCD, a liquid crystal display element or a thin-film magnetic head, a method of exposure implemented by utilizing this exposure apparatus and a method for manufacturing a semiconductor device.
2. Description of the Related Art
Keeping pace with the increasingly higher integration achieved for semiconductor devices, significant progress has been made in the area of exposure apparatuses employed during the photolithography process that is crucial in the fabrication of semiconductor devices. The resolving power achieved by a projection optical system mounted at an exposure apparatus is expressed through the relational expression R=kxc3x97xcex/NA, known widely as Rayleigh""s formula. In this relational expression, R represents the resolving power of the projection optical system, xcex represents the wavelength of the exposing light, NA represents the numerical aperture at the projection optical system and k represents a constant which is determined by process-related factors as well as the resolving power of the resist.
The resolving power required of the projection optical system to support higher integration in the semiconductor device may be achieved by reducing the wavelength of the light from the exposing light source or by increasing the numerical aperture at the projection optical system as the relational expression above indicates. Thus, continuous efforts to achieve a higher NA value have been made. In recent years, by using an exposure apparatus that employs an argon fluoride excimer laser (ArF excimer laser) having an output wavelength of 193 nm as an exposing light source, fine processing down to 0.18 xcexcmxcx9c0.13 xcexcm has become possible.
Since there are at present only two materials, i.e., synthetic quartz glass and calcium fluoride (fluorite), that may be used to constitute the lenses while achieving a satisfactory transmittance in the wavelength range of the output wavelength (193 nm) of the argon fluoride excimer laser, tireless efforts are being made to develop an optical material achieving sufficient transmittance and sufficient internal consistency to be used in this type of exposure apparatus. Currently, synthetic quartz glass achieves an internal transmittance of 0.995/cm or higher, and calcium fluoride has reached a point at which the level of internal absorption can be disregarded.
In addition, the intense efforts made in development of materials to constitute an anti-reflection film coated on the surfaces of optical members are beginning to show (tangible results and at present.) the levels of losses at the individual lens surfaces have been lowered to 0.005 or less.
In the wavelength range of ArF excimer laser light, problems occur in that moisture and organic matter may become adhered to the surfaces of the optical elements constituting the optical systems (the illumination optical system and the projection optical system) in the exposure apparatus to lower the transmittances at the optical systems and in that the glass material itself that constitutes the optical elements becomes degraded due to laser irradiation to result in poor transmittances at the optical systems. The first problem is caused by gases within the spaces enclosed by a plurality of optical elements, or moisture or organic matter originating from the inner walls or the like of the lens barrel supporting the optical systems becoming adhered to the surfaces of the optical systems. The second problem, on the other hand, is attributable to the degrading phenomenon that takes place at the glass material itself while laser light is irradiated at the glass material.
FIG. 13 illustrates time-varying transmittance characteristics in an optical system. The figure presents the optical system transmittance, which represents the ratio of the illuminance of the exposing light between the laser light source and the mask and the illuminance of the exposing light on the wafer measured over specific intervals while irradiating pulse laser light continuously from the laser light source during the laser irradiation and is calculated for each measuring time point. As FIG. 13 indicates, the transmittance at the optical system temporarily becomes lowered immediately after the start of the laser light irradiation (to be referred to as a short-term fluctuation of the transmittance since it occurs over a short period of time relative to the overall change in the transmittance) and then the transmittance gently rises to reach a near saturated state after a certain period of time (to be referred to as a long-term fluctuation of the transmittance which occurs gradually over a long period of time within the overall transmittance change). The degradation occurring at the glass material constituting the optical system causes the transmittance of the optical system to become lowered shortly after the start of the laser light irradiation. Whereas the transmittance at the optical system recovers at a large time constant after the laser light irradiation starts since moisture and organic matter adhering to the surfaces of the optical system are gradually removed from the optical system surfaces through the laser light irradiation. This phenomenon is known as optical cleaning.
It is desirable to expose a photosensitive substrate in a state in which the transmittance at the optical system does not fluctuate greatly, in order to achieve good control of the exposure quantity on the substrate. Accordingly, a near saturated state may be achieved for the transmittance by irradiating the exposing laser light over a specific period of time prior to an exposure operation. However, if an exposure operation is performed following such irradiation, a reduction in the throughput occurs, and, in addition, since the laser is oscillated over a long period of time prior to the exposure operation, the durability of the laser light source becomes lowered.
A first object of the present invention is to provide a method for exposure and an exposure apparatus that maintain the illuminance of exposing light on a photosensitive substrate (exposure object) at a target value at all times, unaffected by the change in the attenuation rate of the exposing light at an optical system occurring over time.
A second object of the present invention is to provide a method for manufacturing a semiconductor device that achieves an improvement in the yield by estimating the change in the attenuation rate of the exposing light at an illumination optical system or a projection optical system occurring over time when exposing the circuit pattern or the like onto a semiconductor substrate.
In order to achieve the objects described above, a method for exposure according to the present invention comprises a step in which a first attenuation rate of the exposing light passing through an optical system is measured before transferring an image of a pattern illuminated with the exposing light onto a first specific surface via the optical system provided within the optical path of the exposing light, a step in which a second attenuation rate of the exposing light passing through the optical system is measured after the image of the pattern is transferred onto the first specific surface, a step in which a third attenuation rate of the exposing light passing through the optical system is measured before transferring an image of the pattern onto a second specific surface different from the first specific surface and a step in which the exposure quantity achieved with the exposing light at the second specific surface is controlled based upon the first, second and third attenuation rates.
During the step for controlling the exposure quantity, time-varying change occurring in the optical characteristics of the optical system may be estimated based upon the first attenuation rate and the second attenuation rate and the estimated time-varying change to occur may be corrected based upon the third attenuation rate.
An estimation of the time-varying change in the optical characteristics to occur may be made by correcting the first attenuation rate and the second attenuation rate through approximation.
The estimated time-varying change in the optical characteristics to occur may be corrected by correcting the difference between the second attenuation rate and the third attenuation rate.
The optical system includes an illumination optical system that illuminates a mask at which the pattern is formed and a projection optical system that projects an image of the pattern onto the first or second specific surface.
The first specific surface is a photosensitive surface of an (nxe2x88x921)th substrate, whereas the second specific surface is a photosensitive surface of an nth substrate.
A method for exposure according to the present invention comprises a first control step in which the exposure quantity achieved with the exposing light at a specific surface is controlled by estimating the attenuation rate of the exposing light passing through an optical system positioned in the optical path of the exposing light and a second control step different from the first control step, in which the exposure quantity achieved with the exposing light at the specific surface is controlled by estimating the attenuation rate of the exposing light passing through the optical system, with an image of a pattern illuminated with the exposing light transferred onto the specific surface via the optical system, at least, either through the first control step or the second control step.
In the first control step, the exposure quantity achieved with the exposing light at a second specific surface may be controlled based upon a first attenuation rate of the exposing light passing through the optical system before an image of the pattern is transferred onto a first specific surface, a second attenuation rate of the exposing light passing through the optical system after the image of the pattern is transferred onto the first specific surface and a third attenuation rate of the exposing light passing through the optical system before an image of the pattern is transferred onto the second specific surface which is different from the first specific surface. In the second control step, the exposure quantity achieved with the exposing light at a third specific surface may be controlled based upon the attenuation rate of the exposing light passing through the optical system before an image of the pattern is transferred onto the third specific surface and the attenuation rate of the exposing light passing through the optical system after the exposing light is irradiated on the optical system over a specific period of time.
In the second control step, the attenuation rate of the exposing light passing through the optical system during the irradiation at the optical system that takes place over the specific period of time may be measured to control the exposure quantity based upon the attenuation rates before and after the irradiation implemented over the specific period of time and the attenuation rate measured during the irradiation.
In the second control step, the length of the specific period of time may be varied in correspondence to the type of mask at which the pattern is formed.
The second control step is implemented until a specific period of time elapses after the exposure apparatus starts an exposure operation, whereas the first control step is implemented after the specific of length of time has elapsed.
The second control step may be implemented when the exposure apparatus is started up after an exposure operation has been stopped over a specific period of time.
In the second control step, a light receiver which receives the exposing light on substantially the same plane as the plane of the specific surface is provided, the light receiver receives the exposing light via the optical system while moving the mask at which the pattern is formed along a specific direction and the exposure quantity achieved with the exposing light at a specific surface may be controlled based upon the results of light reception at the light receiver before the mask is moved and the results of light reception at the light receiver during or after the mask movement.
The specific period of time mentioned above may correspond to the period of time over which at least a single shot area at the photosensitive substrate is exposed.
A method for exposure according to the present invention comprises a step in which exposing light is irradiated on an optical system over a specific period of time while moving a mask at which a pattern is formed along a specific direction before an image of the pattern illuminated with the exposing light is transferred onto a specific surface via the optical system provided within the optical path of the exposing light and a step in which the exposure quantity achieved with the exposing light irradiated onto the specific surface is controlled during the transfer based upon the attenuation rates of the exposing light passing through the optical system before and after the irradiation that takes place over the specific period of time.
A method for exposure according to the present invention includes a step in which a light receiver that receives the exposing light on substantially the same plane as the plane of the specific surface onto which an image of a pattern illuminated with the exposing light is transferred via the optical system provided in the optical path of the exposing light is provided, the light receiver receives the exposing light via the optical system while moving a mask at which the pattern is formed along a specific direction and the exposure quantity achieved with the exposing light at a specific surface is controlled based upon the results of light reception at the light receiver prior to the mask movement and the results of light reception at the light receiver during or after the mask movement.
A method for exposure according to the present invention comprises a step in which exposing light having passed through the optical axis of an optical system provided in the optical path of the exposing light and exposing light having passed through a space outside the axis are received at a light-receiving unit at the same time and the attenuation rate of the exposing light passing through the optical system is measured based upon the results of light reception at the light-receiving unit and a step in which the exposure quantity achieved with the exposing light at a specific surface onto which an image of a pattern illuminated with the exposing light is transferred is controlled in correspondence to the attenuation rate of the exposing light passing through the optical system.
A method for exposure according to the present invention comprises a step in which the attenuation rate of the exposing light passing through an optical system which projects an image of a pattern illuminated with exposing light emitted from an exposing light source onto a photosensitive substrate is measured at a plurality of different time points and a step in which the exposure quantity achieved with the exposing light at the substrate is controlled based upon a plurality of sets of attenuation rate data. In the step for measuring the attenuation rates, either a first light reception method, in which exposing light having passed through the optical axis of the optical system is received or a second light reception method in which exposing light having passed through the optical axis of the optical system and exposing light having passed through a space outside the optical axis are received, is selected and the attenuation rates of the exposing light passing through the optical system are measured based upon the results of light reception implemented through the selected light reception method, and in the step for controlling the exposure quantity, the exposure quantity is controlled in conformance to the attenuation rates of the exposing light passing through the optical system.
An exposure apparatus according to the present invention comprises a measuring device that measures a first attenuation rate of the exposing light passing through an optical system before an image of a pattern illuminated with the exposing light is transferred onto a first specific surface via the optical system provided in the optical path of the exposing light, a second attenuation rate of the exposing light passing through the optical system after the image of the pattern is transferred onto the first specific surface and a third attenuation rate of the exposing light passing through the optical system before an image of the pattern is transferred onto a second specific surface which is different from the first specific surface and a control circuit that controls the exposure quantity achieved with the exposing light at the second specific surface based upon the first, second and third attenuation rates.
An exposure apparatus according to the present invention comprises an estimating circuit that estimates an attenuation rate of the exposing light passing through an optical system at least either through a first estimating method, in which the attenuation rate of the exposing light passing through the optical system provided in the optical path of the exposing light is estimated, or a second estimating method for estimating the attenuation rate of the exposing light passing through the optical system, which is different from the first estimating method and a control circuit that controls the exposure quantity achieved with the exposing light at a specific surface onto which an image of a pattern illuminated with the exposing light is transferred via the optical system, based upon the attenuation rate of the optical system that has been estimated by the estimating circuit.
A method for manufacturing a semiconductor device according to the present invention comprises a step in which a first attenuation rate of the exposing light passing through an optical system is measured before transferring an image of a pattern illuminated with the exposing light onto a first specific surface via the optical system provided within the optical path of the exposing light, a step in which a second attenuation rate of the exposing light passing through the optical system is measured after the image of the pattern is transferred onto the first specific surface, a step in which a third attenuation rate of the exposing light passing through the optical system is measured before transferring an image of the pattern onto a second specific surface different from the first specific surface and a step in which the exposure quantity achieved with the exposing light at the second specific surface is controlled based upon the first, second and third attenuation rates.
A method for manufacturing a semiconductor device according to the present invention comprises a first control step in which the exposure quantity achieved with the exposing light at a specific surface is controlled by estimating the attenuation rate of the exposing light passing through an optical system provided within the optical path of the exposing light and a second control step different from the first control step, in which the exposure quantity achieved with the exposing light at the specific surface is controlled by estimating the attenuation rate of the exposing light passing through the optical system, with an image of the pattern illuminated with the exposing light transferred onto the specific surface via the optical system at least either through the first control step or the second control step.